JERUSALEM — Israel bombarded downtown Gaza City on Tuesday and expanded a massive mobilization of reservists, vowing punishing retaliation against the Hamas militant group that increasingly left residents of the tiny Palestinian territory with nowhere to go.

Four days after Hamas militants stormed into Israel, bringing gunbattles to its streets for the first time in decades, Israel’s military said Tuesday morning that it had regained effective control over its south and the border.

The war has already claimed at least 1,600 hundred lives on both sides — and perhaps many hundreds more. Israel has also said that Hamas and other militant groups in Gaza are also holding more than 150 soldiers and civilians hostage.

The conflict is only expected to escalate from here. Israel expanded the mobilization of reservists to 360,000 on Tuesday, according to the country’s media.

And one major question is whether it will launch a ground offensive into Gaza — a tiny strip of land wedged among Israel, Egypt and the Mediterranean that is home to 2.3 million people and has been governed by Hamas since 2007.

While U.N. agencies appealed for a humanitarian corridor to bring in food and medical supplies, the Israeli military said it struck hundreds of targets overnight in Gaza City’s Rimal neighborhood, an upscale district that is home to ministries of the Hamas-run government, as well as universities, media organizations and the offices of aid organizations.

Palestinian Civil Defense forces pulled Abdullah Musleh out of his basement together with 30 others after their apartment building was flattened by the airstrikes.

“I sell toys, not missiles,’’ the 46-year-old said, weeping. “I want to leave Gaza. Why do I have to stay here? I lost my home and my job.”

After hours of nonstop attacks, residents left their homes at daybreak to find some buildings torn in half by airstrikes and others reduced to mounds of concrete and rebar. Cars were flattened and trees burned out on residential streets that had been transformed into moonscapes.

The devastation in Rimal signaled what could be a new Israeli tactic: warning civilians to leave certain areas and then hitting those areas with unprecedented intensity. On Tuesday afternoon, the military began issuing a new warning for residents of another neighborhood in Gaza City to evacuate.

If these airstrikes continue, Gaza’s civilians will have fewer and fewer places to shelter as more neighborhoods become uninhabitable — and they may not be able to flee either.

Since Hamas took control, Israel and Egypt have severely restricted the flow of goods into the territory and the movement of people in and out.

Israel Says It Will Lay Total Siege to Gaza

Now Israel is saying it will lay total siege to Gaza, cutting off all fuel, food and electricity. Meanwhile, Hamas said Tuesday that Israeli strikes had made the Rafah crossing into Egypt — the only other way out with the Israeli side sealed — impassable.

The bombardments and Israel’s threats to topple Hamas sharpened questions about the group’s strategy and objectives. Hamas leaders have not spoken publicly about whether they anticipated Israel’s ferocious retaliation — and the potential risk of losing much of the group’s government infrastructure — when they launched the weekend attack.

In response to Israel’s aerial attacks, the spokesman of Hamas’ armed wing, Abu Obeida, said Monday night that the group will kill one Israeli civilian captive any time Israel targets civilians in their homes in Gaza “without prior warning.”

Israeli Foreign Minister Eli Cohen warned Hamas against harming any of the hostages, saying, “This war crime will not be forgiven.” Netanyahu appointed a former military commander to manage the hostage and missing persons crisis.

The U.N. said Tuesday that more than 187,000 people have left their homes in Gaza — the most since a 2014 air and ground offensive by Israel uprooted about 400,000.

UNRWA, the U.N. agency for Palestinian refugees, is sheltering more than 137,000 people in schools across the territory. Families have taken in some 41,000 others.

The Israeli military said more than 900 people already have been killed in Israel. In Gaza, 765 people have been killed, according to authorities there; Israel says hundreds of Hamas fighters are among them. Thousands have been wounded on both sides.

The bodies of roughly 1,500 Hamas militants have been found on Israeli territory, but it wasn’t immediately clear whether the numbers supplied Tuesday by the military overlapped with deaths previously reported by Palestinian authorities.

The surprise weekend attack by Hamas left a death toll unseen since the 1973 war with Egypt and Syria — and even those deaths happened over a longer period of time. The weekend attack was also notable for the high number of civilians killed.

That fomented calls to crush Hamas no matter the cost, rather than continuing to try to bottle it up in Gaza. Israel is run by its most hard-right government ever, dominated by ministers who adamantly reject Palestinian statehood.

Israeli Defense Minister Yoav Gallant on Monday ordered a “complete siege” on Gaza.

Jan Egeland, secretary general of the Norwegian Refugee Council aid group, warned that a siege would spell “utter disaster” for Gazans.

“There is no doubt that collective punishment is in violation of international law,” he told The Associated Press. “If and when it would lead to wounded children dying in hospitals because of lack of energy, electricity and supplies, it could amount to war crimes.”

Hamas says it is ready for a long battle against Israel. Desperation has grown among Palestinians, many of whom see nothing to lose under unending Israeli control and increasing settler depredations in the West Bank, the blockade in Gaza and what they see as the world’s apathy.